Europe has clashed with the US Obama administration over climate change in a potentially damaging split that comes ahead of crucial political negotiations on a new global deal to regulate greenhouse gas emissions.

The Guardian understands that key differences have emerged between the US and Europe over the structure of a new worldwide treaty on global warming. Sources on the European side say the US approach could undermine the new treaty and weaken the world's ability to cut carbon emissions.

The treaty will be negotiated in December at a UN meeting in Copenhagen and is widely billed as the last chance to save the planet from a temperature rise of 2C or higher, which the EU considers dangerous.

"If we end up with a weaker framework with less stringent compliance, then that is not so good for the chances of hitting 2C," a source close to the EU negotiating team said.

News of the split comes amid mounting concern that the Copenhagen talks will not make the necessary progress.

Ban Ki-moon, the UN general secretary, told the Guardian last night that negotiations had stalled and need to "get moving".

Ahead of an unprecedented UN climate change summit of almost 100 heads of government in New York next week, Moon said the leaders held in their hands "the future of this entire humanity".

He said: "We are deeply concerned that the negotiation is not making much headway [and] it is absolutely and crucially important for the leaders to demonstrate their political will and leadership."

The dispute between the US and Europe is over the way national carbon reduction targets would be counted. Europe has been pushing to retain structures and systems set up under the Kyoto protocol, the existing global treaty on climate change. US negotiators have told European counterparts that the Obama administration intends to sweep away almost all of the Kyoto architecture and replace it with a system of its own design.

The US distanced itself from Kyoto under President Bush because it made no demands on China, and the treaty remains political poison in Washington. European negotiators knew the US would be reluctant to embrace Kyoto, but they hoped they would be able to use it as a foundation for a new agreement.

If Kyoto is scrapped, it could take several years to negotiate a replacement framework, the source added, a delay that could strike a terminal blow at efforts to prevent dangerous climate change. "In Europe we want to build on Kyoto, but the US proposal would in effect kill it off. If we have to start from scratch then it all takes time. It could be 2015 or 2016 before something is in place, who knows."

Europe is unlikely to stand up to the US, the source added. "I am not sure that the EU actually has the guts for a showdown and that may be exactly the problem." The US plan is likely to anger many in the developing world, who are keen to retain Kyoto because of the obligations it makes on rich countries.

Under Kyoto, greenhouse gas reductions are subject to an international system that regulates the calculation of emissions, the purchase of carbon credits and contribution of sectors such as forestry. The US is pushing instead for each country to set its own rules and to decide unilaterally how to meet its target.

Legal experts say the phrase is designed to protect the US from being forced to implement international action it does not agree with. Farhana Yamin, an environmental lawyer with the Institute of Development Studies, who worked on Kyoto, said: "It seems a bit backwards. The danger is that the domestic tail starts to wag the international dog."

The move reflects a "prehistoric" level of debate on climate change in the wider US, according to another high-ranking European official, and anxiety in the Obama administration about its ability to get a new global treaty ratified in the US Senate, where it would require a two-thirds majority vote. The US has not ratified a major international environment treaty since 1992 and President Clinton never submitted the Kyoto protocol for approval, after a unaminous Senate vote indicated it would be rejected on economic grounds.

The US proposal for unilateral rule-setting "is all about getting something through the Senate," the source said. "But I don't have the feeling that the US has thought through what it means for the Copenhagen agreement."

The move could open loopholes for countries to meet targets without genuine carbon cuts, they said. Europe is not concerned that the US would exploit such loopholes, but it fears that other countries might.

The US State Department, which handles climate change, would not comment.

Stuart Eizenstat, who negotiated Kyoto for the US, said: "There has been a sea change in US attitudes [on climate] and the new president is deeply committed on this issue. But the EU needs to understand the limitations in the US. The reality is that is it impossible for my successor to negotiate something in Copenhagen beyond that which Congress will give the administration in domestic cap-and-trade legislation."

Nigel Purvis, who also worked on the US Kyoto team, said: "It's not welcome news in Europe but the Kyoto architecture shouldn't have any presumed status. Many decisions were taken when the United States was not at the negotiating table. Importing the Kyoto architecture into a new agreement would leave it vulnerable to charges of repackaging."

He denied the US move would weaken the agreement. "It is important for the US to negotiate an agreement it can join, because another agreement that did not involve the United States would set back efforts to protect the climate. Is it weaker to have a system that applies to more countries? I would argue not."